驴子

原帖由 我本善良 于 2008-1-15 15:38 发表

change the heavy side or not

原来不止一个解法啊

我本善良

原帖由 驴子 于 2008-1-15 15:41 发表

原来不止一个解法啊

by 1), we have 3 situation: equals ,left heavier, or right heavier
         let us suggest left heavier
by 2), if still left heavier => bad ball is not in 2 4
         if right heavier   => bad ball is in 2 4

[ 本帖最后由 我本善良 于 2008-1-15 15:48 编辑 ]

驴子

原帖由 我本善良 于 2008-1-15 15:46 发表

by 1), we have 3 situation: equals ,left heavier, or right heavier
         let us suggest left heavier
by 2), if still left heavier => bad ball is not in 2 4
         if right heavier   => bad b ...

你把14和25比的那次，我把124和789比了，也可以

我本善良

原帖由 驴子 于 2008-1-15 15:48 发表

你把14和25比的那次，我把124和789比了，也可以

also

hihigh

原帖由 驴子 于 2008-1-15 13:10 发表

第一次选的时候，对的概率是1/4，错的概率是3/4
第二次，如果不换，对的概率不变，还是1/4，如果换，对的概率就是第一次选错的概率乘以1/2，也就是3/4*1/2=3/8

why 3/8 instead of 1/3?
it's nothing else than the problem: choose 1 out of 3 doors after one of them is eliminated
since the decision this time is to be made under new condition, the original probabilities 1/4, 3/4 should be updated

if there's no right to alter the choice, the probabilitiy of a right decision remains since the elimination of one is not a random process and thus doesn't contributes to the result

我本善良

原帖由 驴子 于 2008-1-15 15:48 发表

你把14和25比的那次，我把124和789比了，也可以

or 123 with 789 in the same way

我本善良

原帖由 hihigh 于 2008-1-15 15:54 发表


why 3/8 instead of 1/3?
it's nothing else than the problem: choose 1 out of 3 doors after one of them is eliminated
since the decision this time is to be made under new condition, the original p ...

the problem is that, you have changed your decision

驴子

原帖由 我本善良 于 2008-1-15 15:54 发表

or 123 with 789 in the same way

这个不行，如果123和789比下来一样就没辙了

我本善良

原帖由 驴子 于 2008-1-15 15:56 发表

这个不行，如果123和789比下来一样就没辙了

but you know the bad ball is heavier or lighter than the others

驴子

原帖由 hihigh 于 2008-1-15 15:54 发表


why 3/8 instead of 1/3?
it's nothing else than the problem: choose 1 out of 3 doors after one of them is eliminated
since the decision this time is to be made under new condition, the original probabilities 1/4, 3/4 should be updated

if there's no right to alter the choice, the probabilitiy of a rightdecision remains since the elimination of one is not a random processand thus doesn't contributes to the result

和1个里面选3个不一样，因为这个是分了两步的，第二步的选择并不是独立于第一步的选择，因此不能简单的用1/3来算
第二步简单的看起来是3个选1个，其实由4个中的哪3个来选择是由你第一步选哪个来决定的，你可以用穷举法来验证一下